forces and the laws of motion -...

Chapter 4

Forces and the Laws of Motion

Assumption College

Mr. Steve

Section 1: Changes in Motion◦ Force

◦ Force Diagrams

Section 2: Newton’s First Law◦ Inertia

◦ Equilibrium

Section 3: Newton’s Second and Third Laws◦ Newton’s Second Law

◦ Newton’s Third Law

Section 4: Everyday Forces◦ Weight

◦ The Normal Force

◦ The Force of FrictionChapter 4: Forces | Physics | Mr. Steve

Chapter 4 Preview

CHANGES IN MOTION

Chapter 4.1

Chapter 4: Forces | Physics | Mr. Steve

Assumption College

Mr. Steve

ryChapter 4.1

Changes in Motion

What is a force?

◦ an action exerted on an object which may

change the object’s state of rest or motion

What is a newton (N)?

◦ the SI unit for force

What does exert mean?

to put forth or into

use, to push/pull on

Whenever there is an interaction between

two objects, there is a force upon each of

the objects. When the interaction ceases, the

two objects no longer experience the force.

Forces only exist as a result of an

interaction.

Chapter 4.1

Changes in Motion

Chapter 4.1

Changes in Motion

In general, a force is any influence that causes

an object to experience a change in:

1. speed

2. direction

3. shape

Chapter 4.1

Changes in Motion

Forces can cause accelerations

p. 118

Chapter 4.1

Changes in Motion

Forces describe the interactions between an

object and its environment

Chapter 4.1

Changes in Motion

Forces exerted on an object can change

the object’s velocity with respect to time.

p. 119

ryChapter 4.1

Changes in Motion

Forces can act through contact or at a

distance

What is a contact force?

◦ a force between two objects that are in

contact with each other

What is a field force?

◦ a non-contact force that acts over a distance,

such as gravity or magnetic

attraction/repulsion

Chapter 4.1

Changes in Motion

Fundamental Forces

StrongForce that holds nuclei together

WeakForce involved in the transmutation

of quarks. (Quantum physics)

ElectromagneticForce between electric charges,

between magnets

GravityForce of attraction between masses

NUCLEAR FORCES

Which one do you think is the weakest?

Which is the strongest? Strong InteractionWhich is the weakest? GravityWhich ones have the greatest range?Gravity &

Electromagnetic –

infinite range!

Chapter 4.1

Changes in Motion

part of the

electromagnetic force

p. 119

Chapter 4.1

Changes in Motion

Force is a vector quantity

◦ example: pushing open a door

When applying a force, both magnitude and direction matter!

ryChapter 4.1

Changes in Motion

What is normal force?

◦ the component of a contact force that acts on

a surface in a direction perpendicular to the

surface

The normal force is what prevents us

from sinking through the chair, the floor,

the ground, etc.

Fn is an action-reaction force pair of an object's

weight

larperpendicunormal

Chapter 4.1

Changes in Motion

ryChapter 4.1

Changes in Motion

What is equilibrium?

1. a state in which opposing forces are balanced

2. a state in which the net force on an object is

Equilibrium

◦ all forces are balanced

◦ there is no acceleration

◦ velocity is constant

could be at rest, 0

could be moving

ryChapter 4.1

Changes in Motion

A free-body diagram helps us study a

situation

What is a free-body diagram?

◦ is a pictorial representation to analyze all the

forces acting on an object

A book sits on a table.1) Draw a simple representation.

◦ e.g., a box or a circle

2) Draw and label all forces.

i. Is gravity acting on the body?

ii. Is there a normal force?

iii. Is anything applying a force?

iv. Any frictional forces?

3) Check, should the body be in

equilibrium?

Steps to drawing a Free-Body Diagram

Some textbooks may use

mg or W instead.

vectors originate from the

center of mass

Chapter 4.1

Changes in Motion

Recall: What’s a system?

p. 120

Force Diagram Free-Body Diagram (of car)

1. A book is at rest on a tabletop.

Chapter 4: Forces | Physics | Mr. SteveEx

Chapter 4.1

Free-Body Diagrams

2. An acrobat is suspended motionless

from the ceiling by a rope.

Chapter 4.1

Free-Body Diagrams

1) Draw a simple representation.

◦ e.g. a box or a circle

3) Check, should the body be in equilibrium?

3. An egg is free-falling from a nest in a

tree. Neglect air resistance.

Chapter 4.1

Free-Body Diagrams

1) Draw a simple representation.

◦ e.g. a box or a circle

3) Check, should the body be in equilibrium?

4. An egg is falling (not freely, do not

neglect air resistance) from a nest in a

Chapter 4.1

Free-Body Diagrams

force of air resistance

5. A rightward force is applied to a book in

order to move it across a desk with a

rightward acceleration. Consider

frictional forces. Neglect air resistance.

Chapter 4.1

Free-Body Diagrams

force applied

appFforce of friction

6. A rightward force is applied to a book in

order to move it across a desk at

constant velocity. Consider frictional

forces. Neglect air resistance.

Chapter 4.1

Free-Body Diagrams

7. A Bangkok taxi is (actually) stopping at a

red light.

Chapter 4.1

Free-Body Diagrams

8. A skydiver is descending with a constant

velocity. Of course, consider air

resistance.

Chapter 4.1

Free-Body Diagrams

9. A car is parked on a sloped street.

Chapter 4.1

Free-Body Diagrams

, cosg yn gF FF

,g yF Check:

Is Fn < Fg?

Is Fn = Fg,y?

fF, sing xf gF FF

9. A car is parked on a sloped street.

10. A hot air balloon is accelerating upward.

Chapter 4.1

Free-Body Diagrams

force of buoyancy

buoyancyF

11. A car is coasting to the right and

slowing down.

Chapter 4.1

Free-Body Diagrams

12. A soccer ball while being kicked at a 45º

angle.

Chapter 4.1

Free-Body Diagrams

13. A soccer ball that has already been

kicked at a 45º angle.

Chapter 4.1

Free-Body Diagrams

14. A race car crashing head-on into a brick

Chapter 4.1

Free-Body Diagrams

Review Sample Problem A on p. 121 then do Practice A on p. 122 (#1-2) at your seats now, independently or in groups of no more than 2

Do in your hw/classwork notebook

SHOW ALL WORK*

(NO WORK = NO CREDIT)

*That means show me that you did the thinking and work. Don’t just show me that you copied!

Chapter 4.1: Changes in Motion

Classwork: Practice A

1. A truck pulls a trailer on a flat stretch of road. The forces acting on the trailer are the force due to gravity (250 000 N downward), the force exerted by the cable connecting the trailer to the truck (20 000 N to the right). The forces acting on the truck are the force due to gravity (80 000 N downward), the force exerted by the road (80 000 upward), the force exerted by the cable (20 000 N to the left), and the force causing the truck to move forward 26 400 N to the right).

a) Draw and label a free-body diagram of the trailer.

b) Draw and label a free-body diagram of the truck.

20000 Ntruck on trailerF

250000 NnF

80000 NgF

80000 NnF

250000 NgF

20000 Ntrailer on truckF

26400 NappF

S I D E V I E W

trailer truck

2. A physics book is at rest on a desk.

Gravitation force pulls the book downward.

The desk exerts an upward force on the book

that is equal in magnitude to the gravitational

force. Draw a free-body diagram of the book.

Formative Assessment

Do Formative Assessment 4.1 p. 122 (#1-6),

independently or in groups of no more than 2

SHOW ALL WORK*

Draw ALL Diagrams!

*That means show me that you did the thinking

and work. Don’t just show me that you copied!

NEWTON’S FIRST LAW

Chapter 4.2

Assumption College

Mr. Steve

Chapter 4.2

Newton’s First Law

An object on which no force is acting will

remain at a constant velocity.

True or False

An object on which no force is acting will always be at rest

ryChapter 4.2

What is inertia?

◦ the tendency of an object to resist being

moved or, if the object is moving, to resist a

change in speed or direction

ความเฉ่ือย -สมบตัขิองเทหวตัถทุีจ่ะคงสภาพน่ิงอยูอ่ยา่ง

เดมิตลอดไป หรอืคงสภาพเคลือ่นทีอ่ยา่งสม่ํา

เสมอในแนวเสน้ตรงตลอดไป

Chapter 4.2

Newton’s 1st law of motion

An object at rest remains at rest, and an object

in motion continues in motion with constant

velocity (that is, constant speed in a straight

line) unless the object experiences a net

external force.The Law of Inertia

If the car were to abruptly stop and the seat belts were not being worn, then the passengers in

motion would continue in motion. Assuming a negligible amount of friction between the

passengers and the seats, the passengers would likely be propelled from the car and be hurled

into the air. Once they leave the car, the passengers become projectiles and continue along a

parabolic trajectory.

Chapter 4.2

notice, the driver’s inertia causes him to fly out of the car

The sum of forces acting on an

object is the net force?

ryChapter 4.2

What is an external force?

◦ a single force that acts on an object as a result

of the interaction between the object and its

environment

What is net force?

◦ a vector sum of all forces acting on an object

◦ a single force whose external effects on a

rigid body are the same as the effects of

several actual forces acting on the body

alternatively…

Chapter 4.2

Force Examples

Internal forces External forces

springF appF

magneticF

electricF

(T = tension)

(R = air resistance/drag)

Chapter 4.2

Mass is a measure of inertia

inertiamass is proportional to

Chapter 4.2

What is equilibrium?

◦ the state in which the net force on an object

is zero

Equilibrium

◦ all forces are balanced

◦ there is no acceleration

◦ velocity is constant

could be at rest, 0

could be moving

Chapter 4: Forces | Physics | Mr. SteveReca

p. 127

buoyancyF

bobonfishF

bobonmanF

Chapter 4.2: Newton’s First Law

Sample Problem B (p. 125)

Yingluck leaves her physics book on top

of a drafting table that is inclined at a 35º

angle.

Find the net force acting on the book.

Step 1: Draw the free-body diagram.

Given:

◦ Fg= 22N

◦ Ff= 11N

◦ Fn= 18N

Wanted:

◦ Fnet= ?

Step 2: Select a coordinate system. Apply.

To simplify the problem, always choose the

coordinate system in which as many forces as

possible lie on the x- and y- axes

Step 3: Find the x- and y-components on

all vectors.

55cos55cos ,

N1355cos)N22(, xgF

55sin55sin ,

N1855sin)N22(, ygF

Step 3: Find the net force in both thex- and y-directions.

for the x-direction for the y-direction

fxgx FFF , nygy FFF ,

)N11(N13xF N18N)18(yF

N2xF N0yF

Yingluck’s book accelerates down the incline

direction in 2 xNFnet

Review Sample Problem B on p. 125 then do Practice B on p. 126 (#1-3) at your seats now, independently or in groups of no more than 2

SHOW ALL WORK*

Classwork: Practice B

Formative Assessment Do Formative Assessment 4.2 p. 127 (#1-5),

independently or in groups of no more than 2

SHOW ALL WORK*

Draw ALL Diagrams!

M4/1: Due _____ (10 points)

M4/2: Due _____ (10 points)

M4/3: Due _____ (10 points)

NEWTON’S SECOND AND

THIRD LAWS

Chapter 4.3

Assumption College

Mr. Steve

Chapter 4.3

Newton’s 2nd and 3rd Laws

Newton’s 2nd law of motion

∑F=m∙a

Chapter 4.3

massforce

onacceleratiforce

if a is constant…

if m is constant…

Chapter 4.3

onacceleratimass

1 1 onacceleratimass

What is the relationship between mass

and acceleration?

or simply…

is inversely proportional to

Chapter 4.3

Newton’s 3rd law of motion

If 2 objects interact, the magnitude of the force

exerted on object 1 by object 2 is equal to the

magnitude of the force simultaneously exerted

on object 2 by object 1, and are opposite in

direction. For every action there is an

equal and opposite reaction.

Chapter 4.3

Check Your Understanding

False—the object with less mass will

experience a greater acceleration.

True or False

Equal forces are applied to 2 objects of different masses,

both objects will experience the same acceleration.

Chapter 4.3

Forces always exist in pairs.

◦ a.k.a. an action-reaction pair

These forces coexist, simultaneously

Chapter 4.3

What about field forces? Do you think they

exist in action-reaction pairs?

Field forces also exist in pairs.

𝐹 =↓ 𝑚 ↑ 𝑎

𝐹 =↑ 𝑚 ↓ 𝑎Force of

MAN on EARTH

Force of

EARTH on MAN

relative to Earth,

man has very small mass

magnitude of

forces are equal

relative to man,

Earth has very large mass

therefore, man’s acceleration is very large

therefore, Earth’s acceleration is very small

Remember:

1. 3rd law states forces are equal and opposite

2. when force is constant, mass is inversely

proportional to acceleration

Chapter 4.3

Conceptual Challenge1. Gravity and Rocks Calculate the force of gravity

on a 2 kg rock. Calculate the force of gravity on a 1 kg rock. The force due to gravity is twice as great on a 2 kg rock as it is on a 1kg rock. Why doesn’t the 2 kg rock have a greater free-fall acceleration?

A greater force acts on the heavier rock, but the heavier rock also has greater mass (greater inertia), so the acceleration is the same.Free-fall acceleration is independent of mass.

p. 130

Remember:

The force of gravity is not constant, but the

acceleration of gravity is constant!

Chapter 4.3

Conceptual Challenge2. Leaking Truck A truck loaded with sand

accelerates at 0.5 m/s2 on the highway. If the driving force on the truck remains constant, what happens to the truck’s acceleration if sand leaks at a constant rate from a hole in the truck bed?

The acceleration will increase as the mass decreases.

p. 130

Review Sample Problem C on p. 129 then do Practice C on p. 130 (#1-5) at your seats now, independently or in groups of no more than 2

SHOW ALL WORK*

Chapter 4.3: Newton’s 2nd and 3rd Laws

Classwork: Practice C

Do Formative Assessment 4.3 p. 132 (#1-5), independently or in groups of no more than 2

SHOW ALL WORK*

Draw ALL Diagrams!

M4/1: Due _____ (10 pts)

M4/2: Due _____ (10 pts)

Chapter 4.3: Newton’s 2nd and 3rd Laws

EVERYDAY FORCES

Chapter 4.4

Assumption College

Mr. Steve

Chapter 4.4

Everyday Forces

This section includes:

1. WEIGHT

2. THE NORMAL FORCE

3. THE FORCE OF FRICTION

Chapter 4.4

Everyday Forces

1. WEIGHT

2. THE NORMAL FORCE

ryChapter 4.4

Everyday Forces

WEIGHT

What is weight?

◦ a measure of the gravitational force exerted

on an object

An object’s weight can change with the

location of the object in the universe

Weight is not an inherent property of an

object like mass.

inherent-

a quality or attribute that is

fundamental, innate to

something or someone

Chapter 4.4

Everyday Forces

WEIGHT

Mathematically, weight is defined as…

or simply…

or sometimes…

preferred notation!

gg amF

Please don’t use this one because we’ll use W

for other things, like…

Chapter 4.4

Everyday Forces

1. WEIGHT

2. THE NORMAL FORCE

THE NORMAL FORCE

What is normal force?

◦ the component of a contact force that acts on

a surface in a direction perpendicular to the

surface

Chapter 4.4

Everyday Forces

Chapter 4.4

Everyday Forces

THE NORMAL FORCE

normal force is always perpendicular to

the contact surface, but NOT always

opposite in direction to gravity or the

applied force.

larperpendicunormal

Chapter 4.4

Everyday Forces

Notice, the

normal force

is not always

equal and

opposite the

force of

gravity.

, cosg ynF mgF

How do we calculate the magnitude of Fn?

Check:

Is Fn < Fg?

Is Fn = Fg,y?

Chapter 4.4

Everyday Forces

What if there is no angle? Is the

equation for Fn still correct?

Calculate the magnitude of Fn .

cosnF mg

cos(0 )nF mg

Chapter 4.4

Everyday Forces

1. WEIGHT

2. THE NORMAL FORCE

ryChapter 4.4

Everyday Forces

THE FORCE OF FRICTION

What is static friction?

◦ the force that resists the initiation (beginning)

of sliding motion between two surfaces that

are in contact and at rest

What is kinetic friction?

◦ the force that opposes the movement of two

surfaces that are in contact and are sliding

over each other

Chapter 4.4

Everyday Forces

Chapter 4.4

Everyday Forces

Chapter 4.4

Everyday Forces

is a variable force; it depends

on the

Kinetic friction is less than (or equal to)

the maximum static friction

When objects accelerate

,maxapp sF F

,maxk sF F

What condition?

Chapter 4.4

Everyday Forces

Friction is not the same for everything.

Friction can be calculated approximately,

using the coefficient of friction.

The coefficient of friction is specific to

each material.

ryChapter 4.4

Everyday Forces

What is the coefficient of friction (µ)?

◦ the ratio of the magnitude of the force of

friction between two objects in contact to the

magnitude of the normal force with which the

objects press against each other

Chapter 4.4

Everyday Forces

F ,maxs

COEFFICIENT OF STATIC FRICTIONCOEFFICIENT OF KINETIC FRICTION

huh??...Why “max”?

Chapter 4.4

Everyday Forces

f nF F

GENERAL EXPRESSION FOR THE FORCE OF FRICTION (using µ)

The force of friction is proportional to the normal force.

What’s the relationship between the force of friction

and the normal force?

Does this make sense?

Chapter 4.4

Everyday Forces

frictional coefficients are determined empirically

empirical-

obtained from observation,

experimentation

Why do you think it’s important to have a low coefficient

of friction for the joints of humans?

Chapter 4.4

Everyday Forces

Friction: imagine two “teethed”

surfaces rubbing together

Chapter 4.4

Everyday Forces

In free-body diagrams, the force of friction is

always parallel to the surface of contact.

The force of kinetic friction is always

opposite the direction of motion.

To determine the direction of the force of

static friction, use the principle of

equilibrium. For an object in equilibrium, the

frictional force must point in the direction

that results in a net force of zero.

Review Sample Problem D on p. 137 then do Practice D on p. 137 (#1-3) at your seats now, independently or in groups of no more than 2

SHOW ALL WORK*

Chapter 4.4: Everyday Forces

Classwork: Practice D

Chapter 4.4

Everyday Forces

Air resistance, or drag, is a form of

friction.

Typical notation:

or…or…

Which shape seems to have the least drag?

known as eddies

ryChapter 4.4

Everyday Forces

What is terminal velocity (vT)?

◦ constant velocity of a falling object when the

upward drag on the object balances the

downward force of gravity.

◦ velocity that occurs at the state of equilibrium

of a falling object with drag

A dropped ball almost immediately reaches terminal velocity.

Pongwiwat and Sasinat are sliding a 225 kg sofa at a constant speed across a wood floor. Pongwiwat pulls with a force of 225 N at an angle of 13º above the horizontal. Sasinat pushes with a force of 250 N at an angle of 23º below the horizontal. What is the coefficient of kinetic friction between the sofa and the floor?

Chapter 4.4

Classroom Practice: Overcoming Friction

coordinate system

Chapter 4: Forces | Physics | Mr. SteveChapter 4: Forces | Physics | Mr. Steve

needed

wanted

Very Important Note!

1. Calculate the x- and y-components of

both of the applied forces

2. Apply the equilibrium principle to find

needed quantities (Ff and Fn)

a) Add the x-components of all forces

b) Add the y-components of all forces

3. Find µk

Chapter 4.4

diagram for Fpull

diagram for Fpush

Fpull,xFpush,x

ple Fpull,y Fpush,y

Chapter 4.4

Apply equilibrium principle

Add up all the forces in the x-direction

Add up all the forces in the y-direction

FnMake the

connection between

Ff and Fk

eqn to use plug in Ff plug in Fn

Final Answer

(units?)

Review Sample Problem E on p. 138-9 then do Practice E on p. 139 (#1-4) at your seats now, independently or in groups of no more than 2

SHOW ALL WORK*

Classwork: Practice E

Do Formative Assessment 4.4 p. 141 (#1-5), independently or in groups of no more than 2

SHOW ALL WORK*

Draw diagrams where necessary!

M4/1: Due _____ (10 pts)

M4/2: Due _____ (10 pts)

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